How does the notification service work? That's what we will see in this video. The network is aware of a large number of events that occur. For example, with respect to user actions, when we have a mobility event due to a UE changing tracking areas, there is a location update that is managed by the AMF. This event is therefore known to the AMF. When a terminal gets turned on or off by a user, there is a registration and deregistration of the UE. These are managed by the AMF, which is therefore aware of them. In the same way, the SMF manages the setup and release of PDU sessions. There are other possible events: for example, the modification of the subscriber profile in the UDM or the modification of the QoS level for a PDU session managed by the PCF. We have a large number of events that are managed by the different NFs. Event notification can open the field to a wide range of value added services. There's a lot of potential. There is a definition of notification services and in order to have them, the NF that wants to be notified must subscribe. We have a subscription procedure and unsubscription procedure. Let's have a look at the general principle, which we have already mentioned a little. We have a consumer NF, which wants to be notified of events occurring in a producer NF. To do this, the consumer NF sends a POST method which is positively acknowledged with a 200 response. When the event occurs in the producer NF, the producer NF is the one that will notify and thus send a post with a response hopefully positive. During this phase, the producer NF acts as an HTTP client and the consumer NF acts as an HTTP server. We have a role reversal. The producer NF becomes the consumer NF and vice versa on the other side. When the event occurs, as we have said, a POST is sent. This POST corresponds to a URI. This URI is linked to a resource on the side of the consumer NF. At the time of the subscription, a callback URI has to be created on the consumer NF side. This callback URI is sent in the initial POST, stored by the producer NF, and as soon as the event occurs, this callback URI that is associated with the POST is the one that is sent by the producer NF. The consumer NF has to be able to stop the subscription if it wishes to. To do this, we will use a DELETE method. But by the same reasoning we need a URI. In this DELETE method, there may be several NF's that have subscribed to notifications from this producer NF. Each subscription must be identified in a unique way. To do this, the producer NF chooses a subscription identity and uses it to create a URI that gets sent in the subscription response. This URI is stored by the consumer NF, and if the consumer NF wants to stop the subscription, it will use this URI that has been created in the request. The resource that corresponds to the subscription gets deleted. Like any subscription, this subscription is created for a limited time. We will therefore add a time limit in the form of an expiration date. There is a date which is requested by the consumer and the date chosen by the producer, which must be sooner or equal to the proposed date. When this date arrives, this corresponds to maximum subscription period and the resource is deleted. Let's look at a slightly more specific example of a notification scenario. The SMF here is looking to be notified of the user profile changes in the UDM. This is a service that is provided by the UDM in the nudm-sdm API, sdm for subscriber data management. This subscription is set up for a given subscriber, a given UE, identified by its SUPI, and then the keyword sdm-subscriptions corresponds to this particular service. We have the callback URI that we have already mentioned, as well as the other fields. The callback URI is stored, the UDM chooses a subscription identity, and this identity is used to create the URI corresponding to the subscription. We reuse the previous request fields and create a child resource, adding the subscription identity. If the SMF wants to stop the subscription, it uses this URI as we can see in the example. When the event occurs, as before, the callback URI is used. In summary, notification services start with a subscription phase. The consumer NF places the callback URI in the subscription requests. The produce rule NF creates a new resource with the subscription ID and places the newly formed URI in the subscription response. When the event requiring notification occurs, the rules change: the producer NF becomes a consumer, and the consumer NF becomes a producer. There are two cases for the end of the subscription, either it can be deleted by a DELETE or there is a timeout.